We can bring together everything that has been said in this chapter about the evolution of life, and some of the earlier remarks about the formation of the planet, into a fourteen-point scenario, a script for one element of the cosmic drama as seen from our planet.

Although the scenario format enables us to describe events in simple declarative sentences without a constant repetition of "probably" and "most likely," bear in mind that this is at best a theory, a consistent set of hypotheses to account for what happened. Some of the statements are on very firm ground, but others are outlines for future research, to which chemists will make a major contribution.

1. The universe began roughly 20 billion years ago, and our galaxy, the Milky Way, approximately 13 billion to 15 billion years ago. Our sun is a second-generation star in this galaxy, formed from the heavy-element-rich debris from earlier stars. As the new star formed by gradual accretion of matter from a dust cloud, local nodes of material in the plane of rotation of the flattened dust cloud began to build up into protoplanets, moving around the sun. One of these aggregates became our Earth.

2. The young Earth was too small to retain whatever original atmosphere it may have had, and what remained was an airless ball of rock, made up mainly of elements, such as silicon, oxygen, and metals, that could form nonvolatile compounds. Heat from compression and from natural radioactive decay caused the interior of the planet to become fluid, leading to the stratification by density that exists today: iron-nickel core, olivine mantle, and a crust of lighter silicates and other minerals.